The present invention relates to a method and an apparatus for inspecting a pattern. The method of the present invention is used, for example, to inspect the pattern formed on a reticle used for producing master masks for producing semiconductor devices.
Photomasks used for producing semiconductor devices are produced by producing a pattern on a reticle approximately 10 times the size of the photomask, reducing the reticle to obtain a master mask, and copying the master mask. The reticle pattern is usually inspected using a microscope. However, such an inspection method requires a considerable number of steps. Further, the accuracy of the inspection is not satisfactory.
It is possible to inspect the reticle pattern using the apparatus illustrated in FIG. 1, which will be introduced later in the brief description of the drawings. The apparatus of FIG. 1 comprises a glass plate 1 having a reticle pattern 11, a light source 2 emitting a light beam L1, an image sensor 3 comprising, for example, a charge coupled device (CCD) receiving light beam L2, a pattern signal conversion circuit 41 for receiving the signal from the image sensor 3, an actual pattern display device 42 comprising, for example, a TV monitor for receiving the signal from the pattern signal conversion circuit 41, a memory device 51 comprising, for example, a magnetic tape, a pattern signal conversion circuit 52 for receiving the signal from the memory device 51, a reference pattern display device 54 comprising, for example, a TV monitor for receiving the signal from the pattern signal conversion circuit 52, a comparator circuit 61 for receiving the signals from the pattern signal conversion circuits 41 and 52, and a memory device 62 comprising, for example, a magnetic tape for receiving the signal from the comparator circuit 61.
In the device of FIG. 1, the actual reticle pattern 11 on the plate 1 is scanned with the light beam L1 emitted from the light source 2 by moving the glass plate 1 in the X' direction from right to left, in the reverse-X' direction from left to right, in the Y' direction by a predetermined amount, in the X' direction from right to left, and then in the reverse-X' direction from left to right, and so on.
The pattern signal produced from the image sensor 3 is converted in the pattern signal conversion circuit 41 to output a signal to the actual pattern display device 42 and the comparator circuit 61. The reference pattern signal is read out from magnetic tape 51 and supplied to the pattern signal conversion circuit 52 which outputs a signal to the reference pattern display device 54 and the comparator circuit 61. The actual pattern and the reference pattern can therefore be visually compared and checked on the display devices 42 and 54.
In the operation of the device of FIG. 1, there exists the problem of missing the corners of the actual reticle pattern 11. That is, the etching process using the photo resist layer as a mask and applied to the metal layer unavoidably causes loss of the corners of the actual reticle pattern. Hence, an actual reticle pattern with missing corner portions is obtained, as illustrated in FIG. 4. The actual reticle pattern 11 of FIG. 4 has round corners. The length C.sub.0 of one of the round corners in the X direction is, for example, less than approximately 1.2 microns. While actual patterns with missing corners of more than 1.2 micron C.sub.0 should be excluded as defective, actual patterns with the missing corners of less than 1.2 micron C.sub.0 should be regarded as permissible even if they do not coincide exactly with the reference pattern.
However, since the reference pattern read out from the magnetic tape 51 is the precisely designed pattern, the reference pattern has no such round corners as in the actual pattern. Therefore, in the device of FIG. 1, it is desired that the result of the comparison between the actual pattern and the reference pattern indicate the actual pattern to be defective only when the length C.sub.0 of the round corner of the actual pattern exceeds a predetermined threshold length, such as 1.2 micron.